Factbox: Oil spill adds to environment insults on Gulf Coast

WASHINGTON (Reuters) - The BP oil spill is the latest in a series of environmental insults to the U.S. Gulf Coast, from wetlands eradication to flood control measures that have starved marshes of new sediment deposits.

WETLANDS CLEARING: Early European settlers cleared coastal swamps and marshes in the Mississippi River delta to control malaria they believed was caused by the fetid air in wetlands. This destroyed coastal wetlands that filter pollution, shelter native species and act as buffers to slow down hurricanes that spawn in the warm waters of the Gulf of Mexico.

RIVER CHANNELIZATION: To keep the Mississippi navigable and protect against floods, the U.S. Army Corps of Engineers channeled the river and built a series of levees along its banks. This prevented the natural variation of the river's course in the lower delta, essentially blocking the formation of new wetlands or the building up of existing wetlands with sediment from the river's vast watershed, which draws from 31 states and two Canadian provinces over 1.2 million square miles (3.1 million square km). Louisiana has lost an estimated 2,000 square miles (5,180 square km) of territory since channelization was authorized in 1928.

SUBSIDENCE AND COMPACTION: Without new sediment deposits to build them up, as would occur if the river followed its natural varying course, coastal wetlands sink and squash down, allowing the salty waters of the Gulf to inundate them. Scientists are looking at this area as a kind of preview of what might happen to other river deltas and low-lying areas if global sea levels rise due to climate change. Species that thrive in marsh or swamp don't necessarily adapt to ocean habitat.

DEAD ZONE: Agricultural chemicals applied to farms throughout the Mississippi watershed flow toward the Gulf of Mexico, and because of the river's channelization, these chemical-laden waters and sediments are shunted away from wetlands and out into the deep Gulf. The nitrogen and phosphorus in fertilizers have created a recurring summer "dead zone" with oxygen levels so low in parts of the Gulf that few species can survive. The 2009 dead zone extended over about 3,000 square miles (7,770 square km), smaller than the average but more deadly because the hypoxic area was closer to the water's surface.

OIL AND GAS EXTRACTION: Drilling for oil and gas in the lower Mississippi delta can accelerate subsidence and compaction by creating empty underground pockets that are ripe to sink down. The pace of this kind of subsidence has slowed because much of the oil and gas has already been extracted; its peak was in the 1970s. Some engineers believe that the constant stream of heavy vehicles on a single two-lane road that leads to Port Fourchon, Louisiana, which supplies offshore drilling, also contributes to subsidence in the area.